The present invention relates generally to locating deviations from a pattern in a simulated aerial image. More specifically, but without limitation thereto, the present invention relates to a method of measuring the quality of a simulated aerial image.
Photolithography is a common technique employed in the manufacture of semiconductor devices. Typically, a semiconductor wafer is coated with a layer of light-sensitive material, called photoresist. Using a patterned mask, or reticle, the semiconductor wafer is exposed to light, typically actinic radiation, projected through the reticle onto the photoresist, which is chemically altered in the areas exposed to the light. The chemically altered areas of the photoresist are removed by chemical etching, leaving a pattern of photoresist lines on the semiconductor wafer that ideally is identical to the reticle pattern. The photoresist pattern is used to create semiconductor devices on the semiconductor wafer.
The degree to which the resulting photoresist pattern corresponds to the reticle pattern is critical to the fabrication of semiconductor devices on the semiconductor wafer. Errors or deviations from the reticle pattern in the photoresist pattern may result in malfunction of semiconductor devices formed on the semiconductor wafer. The shape and proximity of features in the reticle pattern introduce optical proximity effects such as diffraction that may result in errors and deviations in the photoresist pattern.
The variation of light intensity as a function of position in the image plane of the optical projection system used to project the reticle (mask) pattern defines the aerial image. Methods for simulating the aerial image, for example, as described in U.S. Pat. No. 6,171,731, are used in optical proximity correction (OPC) techniques to modify the reticle pattern until the resulting simulated aerial image is within a selected tolerance from the ideal boundary of the chip design. The modified reticle pattern is then used to fabricate a mask for the actual production of semiconductor devices on semiconductor wafers.